1. DNA profile – procedure
2.5.2 – Genetics + Profiling
Genetics: study of the mechanism of inheritance
and variation of traits or characteristics as transmitted from one generation of plants or animals to another .
Species: is a group of animals or plants that can interbreed and produce viable, fertile offspring.
Members of a species share the same characteristics and differ only in minor details.
Heredity: is the study of the natural law or property of organisms whereby their offspring have various physical and mental traits of their parents or ancestors i.e. certain traits are transmitted from one generation to the next.
Genetic information is carried on the DNA molecule as a gene.
Gene: is the unit of heredity found on a chromosome, and is an instruction (code) to the cell to make a particular substance, which helps regulate a trait of an organism, e.g. the gene for tongue-rolling in humans.
There are two possible genes you can have. One gives you the ability to roll your tongue. The other does not give you this ability. These are different forms of the same gene (alleles).
Alleles: Are alternative forms of a gene or a pair of genes found at the same locus / position on homologous chromosomes controlling the same trait.
Gene expression: is the process of changing the information in a gene into a protein and the effect that protein has on the organism.
homozygous: possessing a pair of similar genes for a trait e.g. TT or tt.
heterozygous: possessing a pair of dissimilar genes for a trait e.g. Tt i.e. the dominant and recessive genes.
DNA Structure: DNA is a long, coiled molecule called a double helix.
Composed of two strands of sugars and phosphates – the uprights of the ladder.
Strands are linked by bases – form the rungs.
Four different bases – Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).
Base pairing rule - A always with T, and
C always with G.
Bases can be found in any order along DNA strand – but the order of bases is unique for each DNA molecule.
The order codes for the proteins made by the cell.
Each message for a particular protein is called a gene.
The parts of the DNA that code for proteins are called exons.
Interons: Not all of the DNA carries messages.
The majority of it does not code for proteins and just separates the genes. These non-coding pieces are called junk genes or interons and are highly variable.
Found within or between two genes.
These variable parts of DNA are used when taking a DNA profile (fingerprint).
Triplet base code: DNA has a code that determines the order of amino acids in a protein.
The code is made up of groups of three bases.
Each group codes for a specific amino acid.
DNA profile – procedure
Take a sample of material containing cells e.g. blood or semen.
Extract the DNA from the cell by breaking up the cell and nuclear membranes.
Treat the DNA with special enzymes which recognise specific sequences of bases, usually in the junk genes, and cut the DNA at those sites.
This produces fragments of DNA of varing lengths.
The fragments are placed at one end of a gel.
An electric charge is passed through the gel and the fragments move down the gel.
The smallest pieces move fastest.
The DNA is then transferred onto a nylon membrane
Radioactive DNA probes are put onto the membrane
These attach to the fragments.
The membrane is then put in contact with X-ray film and the distance travelled by the fragments can be seen.
The result is a series of bands similar to bar codes.
Use of DNA profile
Identify criminals – from blood, semen or other tissue left at the scene of a crime.
Identify fathers in paternity cases – the old method of using blood types only proved that a man was not the father.